1. Field of the Invention
The present invention relates to a method and means for compensating for the photomultiplier tube dark signal in a spectrophotometer and, more particularly, to a method and means for reducing the error when compensating for a photomultiplier tube dark signal in a spectrophotometer.
2. Description of the Prior Art
In a spectrophotometer, a beam of light of a known frequency is transmitted through a sample and a photomultiplier tube is positioned to detect the light passing through the sample. The less light absorbed by the sample, the more light is transmitted, and the output of the photomultiplier tube is a current signal proportional to the intensity of the detected light. Thus, the output of the photomultiplier tube is proportional to transmittance.
An operational amplifier is typically used to convert this current signal into a DC voltage signal. Such an operational amplifier typically has inverting and non-inverting inputs and an output whereupon the current from the photomultiplier tube is applied to the inverting input of the operational amplifier. The amplifier produces a DC voltage which is also proportional to transmittance.
Under dark conditions, when there should be no light falling on the photomultiplier tube, there is still a current flowing therethrough, referred to as the dark current, and this dark current produces a proportional voltage at the output of the operational amplifier. If not compensated for, such current and the corresponding output voltage would result in a signal offset and a consequent error when measuring light falling on the tube. Thus, in a spectrophotometer, the signal path from the photomultiplier tube is typically compensated for to reduce the dark voltage to zero.
If the analog signal from the photomultiplier tube is eventually digitized for automatic computer control of dark current offset compensation, an analog-to-digital converter (ADC) must be used. Such an ADC must be capable of converting the entire range of signals of interest during operation of the spectrophotometer and has, by its very nature, a measurement error of .+-.1/2x, where x is the smallest analog step resolvable by the ADC.
Assuming a perfectly nulled dark current offset, then, light signal measurements as digitized by the ADC will be accurate with an error of .+-.1/2x. But, if the same ADC is used to measure the small dark signal for corrective purposes, the dark current compensation may also be in error by .+-.1/2x. This error, combined with the .+-.1/2x error introduced in making later measurements, results in a possible signal measurement error of .+-.x, twice the error of the ADC.